<__msvc_int128.hpp>: Remove unnecessary common_type partial specializations

[frederick-vs-ja]

These partial specializations are unnecessary, because the default mechanisms (the conditional operator and implicit conversions) are already doing right things. It's surprising that the necessity of these specializations hasn't been discussed in #2518.

STL/stl/inc/__msvc_int128.hpp

Lines 1029 to 1043 in 1fe4d43

	#ifdef __cpp_lib_concepts
	template <integral _Ty>
	struct common_type<_Ty, _Unsigned128> {
	using type = _Unsigned128;
	};
	template <integral _Ty>
	struct common_type<_Unsigned128, _Ty> {
	using type = _Unsigned128;
	};
	#else // ^^^ defined(__cpp_lib_concepts) / !defined(__cpp_lib_concepts) vvv
	template <class _Ty>
	struct common_type<_Ty, _Unsigned128> : enable_if<is_integral_v<_Ty>, _Unsigned128> {};
	template <class _Ty>
	struct common_type<_Unsigned128, _Ty> : enable_if<is_integral_v<_Ty>, _Unsigned128> {};
	#endif // ^^^ !defined(__cpp_lib_concepts) ^^^

STL/stl/inc/__msvc_int128.hpp

Lines 1430 to 1444 in 1fe4d43

	#ifdef __cpp_lib_concepts
	template <integral _Ty>
	struct common_type<_Ty, _Signed128> {
	using type = _Signed128;
	};
	template <integral _Ty>
	struct common_type<_Signed128, _Ty> {
	using type = _Signed128;
	};
	#else // ^^^ defined(__cpp_lib_concepts) / !defined(__cpp_lib_concepts) vvv
	template <class _Ty>
	struct common_type<_Ty, _Signed128> : enable_if<is_integral_v<_Ty>, _Signed128> {};
	template <class _Ty>
	struct common_type<_Signed128, _Ty> : enable_if<is_integral_v<_Ty>, _Signed128> {};
	#endif // ^^^ !defined(__cpp_lib_concepts) ^^^

And the non-concept versions (added by me) may do evil - they may opt-out common types when there can be valid ones. Although this is probably not a conformance issue, as when concepts are not available, these integer-class types are just implementation details.

[CaseyCarter]

Good catch! I want to believe these were necessary for some prior formulation of the _Meow128 types, but that's certainly not true for their final form.

[strega-nil-ms]

Good catch!

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for tidying up the STL, and keeping only those partial specializations that bring us joy! 😻 😹 ✨